1. Field of the Invention
This invention relates to an image scanning apparatus, wherein a recording material, such as a photosensitive material, is scanned with a light beam such that an image having been recorded on the recording material may be read out therefrom or such that an image may be recorded on the recording material. This invention particularly relates to an image scanning apparatus, wherein a laser beam produced by a semiconductor laser operating in a single mode at any given time is utilized as the scanning light beam. This invention also relates to a radiation image read-out apparatus for reading out a radiation image by exposing a stimulable phosphor sheet, on which the radiation image has been stored, to stimulating rays, wherein a laser beam produced by a semiconductor laser operating in a single mode at any given time is utilized as the stimulating rays.
2. Description of the Prior Art
Various image read-out apparatuses have heretofore been used wherein a recording material, on which an image has been recorded, is scanned with a light beam which serves as reading light, light radiated out of the portion of the recording material which portion has been exposed to the light beam (i.e. light which has been emitted by the exposed portion of the recording material, light which has passed through the exposed portion of the recording material, or light which has been reflected from the exposed portion of the recording material) is photoelectrically detected, and the recorded image is thereby read out from the recording material. Also, various image recording apparatuses have heretofore been used wherein a light beam, which serves as recording light, is modulated in accordance with an image signal, a recording material (such as a photosensitive material) is scanned with the modulated light beam, and an image represented by the image signal is thereby recorded on the recording material. Ordinarily, in the image read-out apparatus or the image recording apparatus of this type, a single picture element read-out period or a single picture element recording period is defined in accordance with a predetermined picture element clock pulse.
It is considered to utilize semiconductor lasers as scanning light beam sources in image scanning apparatuses for reading out or recording images in the manner described above. A semiconductor laser has various advantages over a gas laser, or the like, in that the semiconductor laser is small in size, cheap and consumes little power. When a semiconductor laser is used in an image scanning apparatus, the image scanning apparatus can be kept compact.
However, it is known that a mode hopping phenomenon occurs with a single-longitudinal-mode semiconductor laser due to a change in the ambient temperature, a change in the drive current, or the like. FIG. 6 shows conditions, under which the mode hopping phenomenon occurs. As illustrated in FIG. 6, the mode of a semiconductor laser is determined by its optical output P and the temperature T. The mode hopping phenomenon occurs at a point at which the mode changes over (e.g., at a point A in FIG. 6). Specifically, in cases where no return light impinges upon the semiconductor laser, the longitudinal mode of the semiconductor laser changes over at random between a mode 1 and a mode 2 at the change-over point A. If the mode hopping phenomenon occurs, the amount of the laser beam produced by the semiconductor laser will fluctuate, or the wavelength of the produced laser beam will fluctuate.
If the mode hopping phenomenon occurs with a semiconductor laser, which is employed as a reading light source in an image read-out apparatus, an image signal, which includes nonuniformity in image density, will be obtained from the image read-out operation. Specifically, for example, as illustrated in FIG. 2, in cases where the recording material, on which an image has been recorded, is a stimulable phosphor sheet described in, for example, U.S. Pat. No. 4,258,264, the sensitivity of the stimulable phosphor sheet depends upon the wavelength of stimulating rays, which cause the sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation. (The sensitivity of the stimulable phosphor sheet is defined by the amount of light, which is emitted by the stimulable phosphor sheet, with respect to a predetermined amount of the stimulating rays.) Therefore, if the wavelength of the stimulating rays serving as the reading light fluctuates, even if the amount of the stimulating rays is constant, the amount of light, which is emitted by the stimulable phosphor sheet when it is exposed to the stimulating rays, will fluctuate. Also, in this case, if the amount of light emission of the semiconductor laser fluctuates due to the mode hopping phenomenon, the amount of the stimulating rays itself will fluctuate.
If the mode hopping phenomenon occurs with a semiconductor laser, which is employed as a recording light source in an image recording apparatus, there will be the risk that nonuniformity in image density occurs in a recorded image due to fluctuations in the amount of the recording light and the dependence of the sensitivity of a recording material upon the wavelength of the recording light.
Particularly, in the case of an image read-out apparatus, as illustrated in FIG. 7, it often occurs that an image signal, which includes nonuniformity in image density that appears as vertical stripes U extending in a sub-scanning direction on an image R, is obtained from the image read-out operation. Studies carried out by the inventors revealed that the nonuniformity in image density appearing as the vertical stripes U is also caused by the mode hopping phenomenon of a semiconductor laser.
A method for coping with the mode hopping phenomenon has been proposed in, for example, Japanese Examined Patent Publication No. 59(1984)-9086. With the proposed method, a radio-frequency current is superimposed upon a drive current for a semiconductor laser, and the single-longitudinal-mode semiconductor laser is thereby caused to oscillate in a multiple longitudinal mode.
However, in such cases, the frequency of the radio-frequency current must be very high and ordinarily on the order of more than 50MHz. A circuit for generating a radio-frequency current having such a markedly high frequency is very expensive. Therefore, if the conventional technique for superimposing a radio-frequency current upon a drive current for a semiconductor laser is applied to an image scanning apparatus, the cost of the image scanning apparatus cannot be kept low.
Also, when the radio-frequency current as described above is used, a high level of radiation noise occurs, and various powerful electromagnetic shields must be used in order to avoid adverse effects of radiation noise. The use of such powerful electromagnetic shields also causes the cost of the image scanning apparatus to become high.